Method of making light emitting diode displays

ABSTRACT

Light emitting diodes (LEDs) and LED bars and LED arrays formed of semiconductive material, such as III-V, and particularly AlGaAs/CaAs material, are formed in very thin structures using organometallic vapor deposition (OMCVD). Semiconductor p-n junctions are formed as deposited using carbon as the p-type impurity dopant. Various lift-off methods are described which permit back side processing when the growth substrate is removed and also enable device registration for LED bars and arrays to be maintained.

This application is a continuation of application Ser. No. 08/165,025filed Dec. 9, 1993, now U.S. Pat. No. 5,453,405, which is a Divisionalof Ser. No. 07/643,552 filed Jan. 18, 1991, now U.S. Pat. No. 5,300,788.

TECHNICAL FIELD

This invention is in the field of light emitting diodes (LEDs).

BACKGROUND OF THE INVENTION

The background to the invention may be conveniently summarized inconnection with four main subject matters. LEDs, LED bars, LED arraysand Lift-off methods, as follows:

LEDs

LEDs are rectifying semiconductor devices which convert electric energyinto non-coherent electromagnetic radiation. The wavelength of theradiation currently extends from the visible to the near infrared,depending upon the bandgap of the semiconductor material used.

Homojunction LEDs operate as follows: For a zero-biased p-n junction inthermal equilibrium, a built-in potential at the junction prevents themajority charge carriers (electrons on the n side and holes on the pside) from diffusing into the opposite side of the junction. Underforward bias, the magnitude of the potential barrier is reduced. As aresult, some of the free electrons on the n-side and some of the freeholes on the p-side are allowed to diffuse across the junction. Onceacross, they significantly increase the minority carrier concentrations.The excess carriers then recombine with the majority carrierconcentrations. This action tends to return the minority carrierconcentrations to their equilibrium values. As a consequence of therecombination of electrons and holes, photons are emitted from withinthe semiconductor. The energy of the released photons is close in valueto that of the energy gap of the semiconductor of which the p-n junctionis made. For conversion between photon energy (E) and wavelength (λ),the following equation applies:${E( {e\quad v} )} = {\frac{1.2398}{\lambda}( {\mu \quad m} )}$

The optical radiation generated by the above process is calledelectroluminescence. The quantum efficiency η for a LED is generallydefined as the ratio of the number of photons produced to the number ofelectrons passing through the diode. The internal quantum efficiencyη_(i) is evaluated at the p-n junction, whereas the external quantumefficiency η_(e) is evaluated at the exterior of the diode. The externalquantum efficiency is always less than the internal quantum efficiencydue to optical losses that occur before the photons escape from theemitting surface. Some major causes for the optical losses includeinternal re-absorption and absorption at the surface. The internalefficiency can exceed 50% and, sometimes, can be close to 100% fordevices made of a very high-quality epitaxial material. The externalquantum efficiency for a conventional LED is such lower than theinternal quantum efficiency, even under optimum conditions.

Most commercial LEDs, both visible and infrared, are fabricated fromgroup III-V compounds. These compounds contain elements such as gallium,indium and aluminum of group III and antimony, arsenic and phosphorus ofgroup V of the periodic table. With the addition of the properimpurities, by diffusion, or grown-in; III-V compounds can be made p- orn-type, to form p-n junctions. They also possess the proper range ofband gaps to produce radiation of the required wavelength and efficiencyin the conversion of electric energy to radiation. The fabrication ofLEDs begins with the preparation of single-crystal substrates usuallymade of gallium arsenide, about 250-350 μm thick. Both p- and n-typelayers are formed over this substrate by depositing layers ofsemiconductor material from a vapor or from a melt.

The most commonly used LED is the red light-emitting diode, made ofgallium arsenide-phosphide on gallium arsenide substrates. An n-typelayer is grown over the substrate by vapor-phase deposition followed bya diffusion step to form the p-n function. Ohmic contacts are made byevaporating metallic layers to both n- and p-type materials. The lightresulting from optical recombination of electrons and holes is generatednear the p-n junction. This light is characterized by a uniform angulardistribution; some of this light propagates toward the front surface ofthe semiconductor diode. Only a small fraction of the light striking thetop surface of the diode is at the proper angle of incidence withrespect to the surface for transmission beyond the surface due to thelarge difference in the refractive indices between semiconductor andair. Most of the light is internally reflected and absorbed by thesubstrate. Hence a typical red LED has only a few percent externalquantum efficiency, that is, only a few percent of the electric energyresults in external light emission. More efficient and thereforebrighter LEDs can be fabricated on a gallium phosphide substrate, whichis transparent to the electroluminescent radiation and permits the lightto escape upon reflection from the back contact. For brighter LEDs,AlGaAs, with the Al percentage equal to 0-38%, grown on GaAs substratesis used. The AlGAs LEDs are usually about 50 μm thick and are grown onGaAs by liquid-phase epitaxy(LPE). The p-n junctions are diffused. Foreven brighter LEDs, the AlGAs layers are grown even thicker (−150 μm),and the GaAs substrates are etched off. The thick AlGAs layer becomesthe mechanical support. With no substrate and a reflector at the backside one can double the external efficiency.

Visible LEDs are used as solid-state indicator lights and as lightsources for numeric and alphanumeric displays. Infrared LEDs are used inoptoisolators, remote controls and in optical fiber transmission inorder to obtain the highest possible efficiency.

The advantages of LEDs as light sources are their small size,ruggedness, low operating temperature, long life, and compatibility withsilicon integrated circuits. They are widely used as status indicatorsin instruments, cameras, appliances, dashboards, computer terminals, andso forth, and as nighttime illuminators for instrument panels andtelephone dials. Visible LEDs are made from III-V compounds. Red,orange, yellow and green LEDs are commercially available. Blue LEDs maybe formed of II-VI materials such as ZnSe, or ZnSSe, or from SiC.

LEDs can also be employed to light up a segment of a large numericdisplay, used for example, on alarm clocks. A small numeric display withseven LEDs can be formed on a single substrate, as commonly used onwatches and hand-held calculators. One of the major challenge for LEDsis to make very efficient LEDs, with high external efficiency.

LED BARS

A linear, one-dimensional array of LEDs can be formed from a linearseries of sub-arrays, wherein the sub-arrays comprise a semiconductordie with several hundred microscopic LEDs. Each LED is separatelyaddressable and has its own bond pad. Such a die is referred to as anLED bar and the individual LEDs in the array are referred to as “dots”or “pixels”.

LED bars are envisioned as a replacement for lasers in laser-printerapplications. In a laser printer, the laser-printer applications. In alaser printer, the laser is scanned across a rotating drum in order tosensitize the drum to the desired pattern, which is then transferred topaper. The use of electronically scanned LED bars for this purpose canresult in replacement of the scanning laser with a linear stationaryarray of microscopic LEDs that are triggered so as to provide the sameoptical information to the drum, but with fewer moving parts andpossibly less expensive electric-optics.

Currently, commercial LED bars are of two types: GaAsP on GaAssubstrates and GaAlAs on GaAs. The GaAsP/GaAs bars are grown by VaporPhase Epitaxy (VPE). Because of the lattice mismatch between GaAsP andGaAs, thick GaAsP layers must be grown of about 50 microns or morethickness and growth time per deposition run is long (5-6 hours). LEDbars produced in this fashion are not very efficient and consume muchpower, and have relatively slow response times.

The second type of LED bar, i.e. GaAlAs/GaAs is grown by Liquid PhaseEpitaxy (LPE). LPE growth is cumbersome and does not lead to smoothgrowth, or thin uniform layers, and is not well suited to the growth ofcomplex structures requiring layers of different III-V compositions.

One of the most important performance requirements for LED bars isdot-to-dot uniformity of the optical output or electroluminescence (orηε). Uniformity of 10 to 15% is currently typical but the marketplacedesire ±2% or better. Another major requirement is output stability overthe lifetime of the LED bar. Currently stability is poor. Anotherimportant feature is high brightness, which is presently not very good.Elimination of wire bonding which is currently not available is alsohighly desirable. Thermal sinking is also important, particularly in thecase of inefficient GaAsP bars, in which the brightness is dependentupon operating temperature.

LED ARRAYS

Currently, arrays of LEDs, addressable in two directions (i.e., an X-Yarray or X-Y matrix), have been formed of discrete LED chips mounted onprinted circuit boards. The resolution of such arrays is limited by thepixel size which is on the order of 200 microns square.

An alternate approach has been to use LED bars to project the light onscanning mirrors. The inclusion of moving parts causes life and speedlimitations.

A need exists, therefore, for a monolithic X-Y addressable array withhigh resolution properties.

LIFT-OFF METHODS

In the fabrication of LEDs, LED bars and LED arrays, it is desirable fora number of reasons, chiefly relating to quantum output efficiency, toutilize thin film epitaxial semiconductor layers for device fabrication.Furthermore, as stated in U.S. Pat. No. 4,883,561 issued Nov. 28, 1989to Gmitter et al.:

“In thin film technology there has always been a tradeoff between thematerial quality of the film and the ease of depositing that thin film.Epitaxial films represent the highest level of quality, but they must begrown on and area accompanied by cumbersome, expensive, bulk singlecrystal wafer substrates. For some time, research has focused on thepossibility of creating epitaxial quality thin films on arbitrarysubstrates while maintaining the ultimate in crystalline perfection.

The main approach has been to attempt to reuse the substrate wafer byseparating it from the epitaxially grown film; however, to undercut avery thin film over its entire area without adversely affecting the filmor the underlying substrate, the selectivity must be extremely high.This is very difficult to achieve. For example, J. C. Fan has describedin Journale de Physique, re, Cl, 327 (1982) a process in which anepitaxial film is cleaved away from the substrate on which it is grown.Such cleavage, at heat, is difficult to achieve without damage to thefilm and/or substrate, or without removal of part of the substrate.Also, in some instances, the cleavage plane (<110>) and the growth plane(<110>) of the film may be mutually exclusive.

In a paper by Konagai et al. appearing in J. of Crystal Growth 45,277-280 (1978) it was shown that a Zn doped p-Ga_(l-x)Al_(x)As layer canbe selectively etched from GaAs with HF. This observation was employedin the production of thin film solar cells by the following techniques.In one technique, zinc doped p-Ga_(l-x)Al_(x)As was grown by liquidphase epitaxy (LPE) on a n-GaAs grown layer on a GaAs single crystalsubstrate. During this LPE growth of the Zn doped Ga_(l-x)Al_(x)As. Indiffuses into the surface of the underlying GaAs to form a p-type GaAslayer and hence p-n GaAs junction. The surface p-Ga_(l-x)Al_(x)As isthen selectively etched away leaving the p-n junction GaAs layers on theGaAs substrate.

In another solar cell fabrication process Konagai et al describe a“peeled film technology,” which will be referred to here as lift-offtechnology. A 5 micron thick Ga_(0.3)Al_(0.7)As film is epitaxiallygrown on a GaAs <111> substrate by LPE. A 30 micron thick Sn dopedn-GaAs layer is then grown over the Ga_(0.3)Al_(0.7)As layer and a p-njunction is formed by diffusing Zn into the specimen utilizing ZnAs₂ asthe source of Zn. Appropriate electrical contacts are then formed on thefilms using known photoresist, etch and plating techniques. The surfacelayer is then covered with a black wax film support layer and the waferis soaked in an aqueous HF etchant solution. The etchant selectivelydissolves the Ga₀₃Al_(0.7)As layer which lies between the thin solarcell p-n junction device layers and the underlying substrate, allowingthe solar cell attached to the wax to be lifted off the GaAs substratefor placement on an aluminum substrate. The wax provides support for thelifted off film.

While the technique described above has been described in the literaturefor over ten years, it was not adopted by the industry. One reason forthis was a difficulty encountered in completely undercutting theGa_(0.3)Al_(0.7)As ‘release’ layer in a reasonable time, especially whenthe area of the film to be lifted-off was large. This difficulty arosedue to the formation and entrapment of gas formed as a reaction productof the etching process, within the etched channel. The gas created abubble in the channel preventing or diminishing further etching andcausing cracking in the epitaxial film. The problem could only bepartially obviated by using very slow reaction rates (very dilute HFsolutions). Since both the time required for lift-off and the risk ofdamage to the overlying film are important, the process was virtuallyabandoned.”

In the Gmitter et al. patent, a lift-off approach was used whichcomprised selectively etching away a thin release layer positionedbetween an epitaxial film and the substrate upon which it grows, whilecausing edges of the epitaxial film to curl upward as the release layeris etched away, thereby providing means for the escape and outdiffusionof the reaction products of the etching process from the area betweenthe film and substrate.

The Gmitter et al. process uses Apiezon (black) wax applied to the frontside layer to be separated. The stress in the wax imparts a curvature tothe layer being separated or lifted, thereby allowing etching fluidaccess to the etching front. This process is inherently limited torelatively small areas. The etching front must commence from the outeredge of the total area being lifted off. This result in long lift-offtimes, for example, up to 24 hours for a 2 cm² area.

In addition, the curvature necessary for lift-off is caused by a lowtemperature wax so that no high temperature processing can be done onthe backside of the lifted area. This results from the fragile nature ofthe thin film which must be supported at all times. The film, whensupported by the wax on the front side, is curved and cannot be furtherprocessed in that shape, without a great deal of difficulty. If the waxis dissolved to allow the film to lay flat, the film must first betransferred to a support by applying the backside surface to a support,in which case, access to the backside is no longer feasible without afurther transfer. Presently, samples are cleaved to size, whichprecludes substrate reuse in full wafer form. Thus, this process isuseful only for individual small areas that do not require backsideprocessing. More importantly, there is no known method of registrationfrom one lifted-off area to another. Thus, large scale processing forLED bars and LED arrays using this technique is not presently practical.

SUMMARY OF THE INVENTION

The invention is directed to novel LEDs and LED bars and arrays, per se.The present invention is also directed to a new and improved lift-offmethod and to LEDs, LED bars and LED arrays may be such method.

LIFT-OFF METHODS

In one embodiment of the novel left-off method, a thin release layer ispositioned between an epitaxial film and the substrate upon which it isgrown. A coating of materials having different coefficients of expansionis applied on the epitaxial film layers. The top structure comprisingthe coating and the epitaxial layers is then patterned as desired toincrease the amount of etchant front by cutting channels to completelylaterally separate individual lift-off areas or by cutting slits partway into the epitaxial film.

The entire structure is then brought to a suitable temperature whichcauses thermal stress between the coating compositions while thestructure is subjected to a release etchant resulting in lift-off ofindividual thin film areas supported by the coating.

Where registration between film areas is desired, such as in thefabrication of LED bars or LED arrays, a coating of material, such asuncured UV epoxy, which is capable of being transformed from a morereadily soluble state to a less soluble state by UV radiation is appliedover a thin film epilayer formed on a release layer over a substrate. AUV light transparent grid with a plurality of openings is affixed overthe transformable coating.

A photomask, with an opaque pattern to cover the openings in the grid,is affixed over the grid. The transformable coating is cured every whereexcept beneath the covered openings by exposing the layer to UV lightthrough the photomask.

The mask is then removed. The uncured portions, i.e., in the openings ofthe grid are then removed by a solvent down to the epitaxial surfaceleaving a cured grid layer of epoxy over the thin film surface.

Next, the epitaxial layer is etched away down to the release layer usingthe openings in the grid to create access for the etchant at the manypoints across the structure.

The thin film layer may then be lifted off while attached to the supportgrid of remaining cured transformable material. The backside may then beprocessed on the wafer (substrate) scale with the wafer registrationstill retained.

In one of several alternative lift-off embodiments, release andregistration is accomplished by forming channels between device areasdirectly on the thin film and thereby exposing area of the release layerbetween lift-off areas. The exposed areas are then filled with etchantmaterial. While the exposed areas are so filled, a lift-off supportstructure, such as UV curable epoxy tape, or other fairly rigidmaterial, is affixed to the frontside of the wafer trapping the etchantin the channels. Eventually, the trapped etchant consumes enough releaselayer material to enable the lift-off support, together with theunderlying lift-off area, to be removed from the underlying substratewith registration intact.

LED AND LED BARS

In accordance with the present invention, thin film epitaxialGaAs/AlGaAs LEDs and LED bars are formed by an Organo-Metallic-ChemicalVapor Deposition (OMCVD) lattice matched process. The p-n junctions aregrown during OMCVD of an active GaAs layer which is sandwiched betweenAlGaAs cladding layers formed on a GaAs or Ga substrate. Preferably,carbon is used for the p-type dopant.

The cladding layers confine injected minority carriers to regions nearthe p-n junction.

A thin top surface of GaAs (light emitting surface) layer of about 1000Å, or less, if formed to assist in current spreading at the pixelregion. Current spreading is desired at the pixel region to provideuniform current through the p-n junction, but is undesirable beyond thepixel region where it would tend to cause a non-uniform pixel boundary,and for ease in contacting the device. The thin top surface layer alsoprevents oxidation of the AlGaAs cladding layer.

Various methods are employed to isolate the LED bar dots from each otherand to preclude current spreading beyond the desired pixel boundary. Onesuch method is ion or proton bombardment to destroy the crystal qualitybetween dot regions and another is etch isolation through to the p-njunction between pixels. A simple but elegant alternate solution to theproblem is to initially grow very thin cladding layers which serve theminority carrier confinement function near the p-n junction region, butare poor lateral conductors due to their thinness and thereby serve toprevent current spreading laterally.

By way of contrast, the currently known art uses a thin cladding layerto spread the current. Also, most LED bars use patterned Zn-diffusedjunctions to define the pixels. In that case, a thick top layer is usedbecause the Zn diffuses quite deeply. This deep diffusion is useful forcurrent spreading, but may not be easily controllable. In the presentinvention, which discloses grown epitaxial junctions, ion implantation,etching, or anodization may be used, as aforesaid, to create the highresistance region between pixels. A fourth approach, outlined above,uses a thin highly conductive patterned GaAs layer for current spreadingto the pixel boundaries, and a thin and much less laterally conductiveAlGaAs cladding layer. The thin GaAs layer (between 500 Å and 1000 Å,and preferably less than 1000 Å) transmits a large fraction of the lightand conducts current to the edges of the pixel, provided the pixel sizeis not too much larger than 30 μm square. Thus, by patterning the GaAs,the current spreading is limited to the edge of the GaAs contact layer,and the cladding layer does not have to be patterned, leading to betterplanarity of the surface, and also avoiding the formation of exposedjunction edges and associated deleterious perimeter leakage currents.

Optionally, the lift-off methods previously discussed may be employed toseparate a front processed LED bar from its substrate, or the backsurface of the substrate may simply be metallized to form a back contactfor current flow.

LED ARRAYS

In accordance with the invention, LED arrays are formed on a suitablesubstrate comprising a III-V epitaxial heterojunction, preferablycomprising AlGaAs cladding layers with a GaAs carbon doped p-n junctionformed between the AlGaAs layers using the OMCVD process describedabove. Optionally, an etch stop or release layer is formed between thesubstrate and the epi-layers when it is desired to separate thesubstrate after front side processing.

A pattern of contact pads and bus bars is then formed on the top (orlight emitting) surface. Next, each LED dot, or pixel, is isolated byetching part way through the epi-layers forming isolated dot mesas.

A planar support structure (preferably of light transparent material,such as glass) is then bonded to the top of the mesas by a suitableadhesive, such as light transparent epoxy.

After the support is attached, the substrate is etched off, or cleavedoff, leaving the LED film patterned on one side (front side) withcontact pads and bus bars attached to the support structure. Theremaining side (called the backside) is exposed when the substrate isremoved. The backside contacts (running orthogonal to the top sidecontacts) and bus bars are then formed by photolithography followed byelectroplating or evaporation of the metal for the contact to form anLED array of LED pixels addressable in two orthogonal directions.

A multicolor array can be formed by two or more such arrays. In themulticolor embodiment, each array is formed with a different bandgapmaterial to create light emissions of different wavelength and, hence,different colors. The larger bandgap material is formed closer to thetop or light emitting surface. The material with the larger bandgap willbe transparent to radiation from the smaller bandgap material.

A “smart” switch can be formed using an x-y LED array, as describedabove. The LED array is mounted inside a light transparent pushbutton.The LED X-Y contacts are addressed by a microprocessor, so that amessage can be displayed on the face of the button indicating, forexample, the button function.

A digital multiplexed infrared (IR) and visible imageconverter/enhancement system can be formed using the previouslydescribed lift-off processes and backside processes to form X-Y arraysof photodetectors and X-Y LED arrays of very thin epi-layers withregistered dots and metallization on both sides.

An image, focused on the X-Y detector arrays, is converted to anelectrical signal by sequentially detecting the charge or current ineach IR detector element. An X-Y photo-detector array, formed as above,is coupled to a microprocessor controlled digital multiplexer comprisingan array of transistor gates.

The detected signal is amplified and drives a corresponding visiblelight emitting dot or pixel in an LED array, resulting in conversion ofthe IR image to a visible light image. The pixel size can be as small as25 microns of even less, depending on the wavelength of the light and upto the layer thickness, i.e., approximately 1 micron, resulting in veryhigh resolution and fairly low cost.

The above summary will now be supplemented by a more completedescription of the invention in the various embodiments described inconnection with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1 d are a series of schematic cross-sectional flow diagramsshowing various steps in a first embodiment of a lift-off process of theinvention.

FIG. 1e is an enlarged view of portion of the LED layer 16 of FIGS. 1a-1d.

FIG. 1f is a cross-sectional view of a single LED formed according tothe process of FIGS. 1a-e.

FIG. 2a is a partial perspective view of a portion of a LED wafer duringlift-off processing, according to a second embodiment of the lift-offprocess.

FIG. 2b is a sectional view taken along lines II—II of FIG. 2a of thelift-off structure after one step in the process.

FIG. 3a is a partial perspective view of a portion of an LED waferduring lift-off processing in an alternate embodiment whereinregistration is maintained.

FIGS. 3b and 3 c show cross-sections of the structure of 3 a afteradditional steps in the process.

FIGS. 4a-4 e are schematic drawings of a wafer during various steps inthe processing flow involved in a further lift-off embodiment of theinvention.

FIGS. 5a-5 b are schematic cross-section drawings of a wafer subjectedto another lift-off procedure in accordance with the invention.

FIGS. 6a-6 d are schematic drawings of a wafer in yet another LEDlift-off process in accordance with the invention.

FIGS. 7a-7 c are schematic sectional drawings of a last embodiment ofthe lift-off method of the invention.

FIGS. 8aa-8 af and 8 ba-8 bk is a process flow digram of the main stepsin fabricating an LED bar in accordance with a mesa etch isolationprocess with a corresponding schematic sectional view of a waferstructure so processed shown beneath each step.

FIG. 9 is a cross-sectional side view of wafer during step k of FIG. 8b.

FIG. 10a -10 h is a process flow diagram of the main steps infabricating an LED bar in accordance with an alternate process with acorresponding schematic sectional view of a wafer structure so processedshown beneath each step.

FIGS. 11aa-11 af and 11 ba-11 b is a process flow diagram of the mainsteps in fabricating an LED bar in accordance with yet another alternateprocess with a corresponding schematic sectional view of a waferstructure so processed shown beneath each step.

FIG. 12a is a plan view of a wafer before being diced into separate LEDbars.

FIG. 12b is a plan view of an LED bar 200 made in accordance with any ofthe processes described in connection with FIGS. 8, 10 or 11.

FIG. 13 is a perspective view of a LED pixel from an X-Y addressable LEDarray embodiment of the invention.

FIGS. 14a-b are schematic sectional views of a wafer being processed toform an X-Y addressable LED array.

FIGS. 14c-e are schematic partial perspectives showing a wafer duringsuccessive additional process steps.

FIG. 15 is a plan view of an X-Y addressable LED array mounted on asilicon substrate with associated silicon electronic circuitry.

FIG. 16 is a side view of a pixel of a tri-color X-Y addressable LEDarray.

FIG. 17 is a plan view of the array of FIG. 16.

FIG. 18 is a schematic side view of an IR to visible light converterembodiment of the invention.

FIG. 19 is a schematic diagram of the converter of FIG. 18.

FIG. 20 is a side view of an alternate embodiment of FIG. 18.

FIG. 21 is a top view of a “Smart” button embodiment in accordance withthe invention.

FIG. 22 is a schematic side view of the button of FIG. 21.

FIG. 23 is a plan view of the top of an LED bar in accordance with theinvention.

FIG. 24 is a schematic side view of FIG. 23 taken along line XXII—XXII.

FIG. 25 is a plan view of a silicon wafer adapted to mate with the LEDof FIGS. 23 & 24.

FIG. 26 is a schematic side view of a silicon structure taken alonglines XXV—XXV of FIG. 25 adapted to mate with the LED bar of FIGS. 23 &24 to form a hybrid circuit.

DETAILED DESCRIPTION OF THE INVENTION

I. LIFT-OFF METHODS

A first embodiment of the invention will now be described in connectionwith item 10 of the cross-sectional drawings of FIGS. 1a-1 f. Asubstrate 12, which may comprise any suitable substrate material uponwhich to grow epitaxial hetero-layers, is provided. A release layer 14is grown, preferably by OMCVD, on substrate 14. Layer 14 is preferablyformed of AlAs for an AlGaAs/GaAs device. For an InP device, an InGaAsrelease layer is preferred. AlAs is preferentially etched by HF acid,while InGaAs is preferentially etched by sulfurix/hydrogen peroxide andwater solution.

There are a number of ways to achieve liftoff depending on whether largecontinuous sheets of material, discrete areas, or registered discreteareas are desired. The first liftoff embodiment is intended to improveupon the prior art for lifting off large continuous areas of material.In this case, curvature is needed to speed up the process. A coating 18& 19 is formed on the front side processes LED structure 16 formed on arelease layer 14 grown on substrate 12 (FIG. 1a). The coating mayconsist of a single film or combination of thick or thin film materialswith various thermal coefficients of expansion. For example, coatings 18& 19 may comprise a nitride, metal, bi-metal or coated thin glass. Thestress in the coating can be tailored for the exact application toimpart a controlled curvature to the thin film of material as it isreleased, thereby assisting the flow of etchant to the liftoff front.

The curvature can be tailored for use at room temperature, in which caseit would need to be removed afterward for further processing withoutcurvature, or it could be tailored to life flat at room temperature, andto impart the compressive stress to the liftoff material at elevated orreduced temperature. In one example, a thin glass sample 18 can becoated with a layer of purposely stressed nitride 19, in order to putthe glass under compression. Stress is induced by varying the rate ofdeposition and by depositing at a temperature which is different thanthe temperature at which the structure will be used. For example, thefilm materials may be sputter at low deposition temperatures. The glasscan be coated after it is affixed to the front of the liftoff material(i.e. LED structure 16) or before. The coating is then patterned and thestructure is then exposed to a release etchant such as, HCL, HF orH₂SO₄/H₂O₂/H₂O using the coating layers 18/19 as a mask. Due to thestress, the glass 18 and thin film sandwich 16 will curve up as thematerial is released from it's rigid substrate, allowing the etchantaccess to the front, FIG. 1c. After complete liftoff, the nitridecoating can be removed, allowing the sample to lay flat again, whilestill being supported by the glass for backside processing.

This process could also be applied for lifting off small area devicesbecause although curvature is not necessary, it can still be beneficialin speeding up the release process. In this case, the coating layer 18plus the LED structure 16 would simply be patterned in the desiredconfiguration, FIG. 1b, using for instance well known photolithographytechniques followed by wet etching, and subsequently removed down to therelease layer 14 as seen in FIG. 1b.

An alternate liftoff process for lifting discrete devices will now bedescribed in connection with FIGS. 2a and b, wherein corresponding itemsin FIG. 1 retain the same reference numeral in FIG. 2. By dividing thetotal wafer area into small areas, or equivalently, bringing theundercutting etchant in contact with many points across the wafer, therequirement for curvature is avoided. Areas as large as 0.5 cm wide havebeen shown to lift readily without curvature. As shown in the partialperspective cross-section of FIG. 2a, a substrate 12 has formed thereona release layer 14, followed by an LED structure 16, as described inconnection with FIG. 1. All front side processing, such as bonding padsand metal contacts (not shown) to the LED structure 16 are completed.

A state-transformable material, that is, a material which can beselectively transformed from one state to another state is formed on, orapplied to, the front side processed LED structure 16. In the originalstate the transformable material may be substantially insoluble and whentransformed may become soluble. The material is selectively transformedwhere desired and the material which is left in its original state isthen removed. For example, a UV curable epoxy 30 may be spread over thestructure 16. This epoxy has the property that exposure to intense UVlight causes it to cure and become a solid. Uncured spray is soluble insolvents such as acetone and trichloroethylene while the cured epoxy isnot. The epoxy is irradiated with UV light in the desired pattern usingfor instance a standard chrome photolithography mask 34 to block thelight where curing is not desired. After exposure, the mask 34 isremoved and the uncured epoxy is removed with a solvent such astrichloroethylene.

Next the LED structure 16 is removed down to the release layer 14. Thecurved epoxy 30 is left on the LED structure to serve as a support forthe thin film LED structure 16 after separation from the substrate. Inthis manner, the etching front is increased by dividing up the totalwafer area of structure 16 into smaller areas by cutting channels 40down to the release layer 14. In this way, a whole wafer's worth ofdevices can be lifted in such shorter times than it would take to lift acontinuous sheet of LED material from the same size wafer, and it isachieved without the need for curvature.

Where registration between LED devices separated from the same wafer isrequired, as in LED bars and arrays, the liftoff method of the alternateembodiment of FIGS. 3a-3 d offers many advantages. Like numerals areused in FIGS. 3a-3 d and subsequent figures for corresponding items inthe previous figures.

This alternate process of FIG. 3 solves the difficult problem of tryingto register small device or pixel areas of material with respect to eachother, while at the same time, allowing the etching medium access to theexposed release layer. The ability to do this allows for easy retrievalfrom the solution, wafer scale processing on the backside, and shortliftoff times due to the smaller areas and maximum etching front. Thisapproach also enables registration of devices throughout the entirewafer area while still providing the etching solution access to manypoints across the wafer.

Turning now to FIG. 3(a), there is shown a rectangular partial sectionof a conventional III-V planar circular 3 inch wafer. The wafer isformed of a semiconductor substrate 12 upon which a release layer 14 isdeposited by OMCVD followed by a front processed LED structure 16, allas previously described above.

Transformable material, such as uncured liquid UV epoxy 50 is spreadonto the top or front surface of structure 16. The point of departurewith the previous embodiment occurs in the next step, when a perforatedplanar grid 52, made of material transparent to UV light such as glass,is aligned on top of the epoxy 50. The perforations 56 extend orthogonalto, and through, the plane of grid 52.

A photomask 58 with opaque areas designed to cover the perforations 56is then positioned over the grid 52 (FIG. 3a). (An optional UVtransparent mask release layer can be placed between the mask 58 and thegrid 52 if intimate contact is desired to facilitate mask removal). UVlight is focused onto the mask, curing the underlying epoxy 50 aeverywhere except beneath the opaque areas 58, as shown in FIG. 3b.Wherein the cured sections of epoxy 50 are shown in shaded section andthe uncured sections are in blank. The mask 58 is removed. The uncuredepoxy 50 is removed from the openings 56 by a suitable solvent and theLED structure 16 etched away through the openings down to the releaselayer 14. The release layer is then etched away using openings 56, asprovided above. Access for the etchant is thus achieved at many pointsacross the wafer, resulting in a large area LED structure attached togrid 52 by cured epoxy 50 a (FIG. 3c).

Another approach to registration is to form channels 60 directly in theLED material by etching down to the release layer 14, thereby formingchannels in the LED material alone (FIG. 4a) . Any other method thatforms channels 60 or access streets between the areas 70 to beseparated, as shown in the plan view of FIG. 4c, could also be usedincluding the previous UV curved epoxy technique.

A support 80 can then be attached to the material 70 over the channels60 and then the etchant can be allowed to run along the channels,thereby giving the etchant access to the center of the wafers (FIGS.4d/4 e). The support materials must be rigid enough that they do no fillin the channels that have been formed and thereby force the etchant cut.As shown in FIGS. 7a-7 c, rigid alumina support 110, combined withdouble-sided UV release tape 120, may be used. One side of the tape 120is adhered to the alumina, and the other side of the tape is adhered tothe front of the structure 16 after the channels have been formed.

These UV release tapes are well known in the industry for dicing andshipping small devices and have proven to be an excellentsupport/adhesive choice in the present application for several reasons.These tapes have the property that when they are exposed to intense UVradiation, they lose most of their adhesion, so they can be easilyremoved. In addition, moisture does not seen to affect the adhesive, andthey can be applied with great success, even if submerged in liquid.

The second, rigid support 110 should be formed from material which istransparent to UV radiation if it will be desired to release the deviceslater. It should also not be attached by the specific etchant used.After the support 110, 120 is applied, the etchant is then allowedaccess to the sample and will run up the channels and undercut thedevices. The devices are then released attached by UV releasable tape120 to the alumina 110. Taller channels may assist in speeding up thecapillary action to achieve faster release times. Other methods may alsobe employed to speed along the movement of the etchant up the channels60, including vacuum assistance, ultrasonic assistance, etc.

Another way to speed up the action of the etchant is to avoid theproblem altogether by placing the etchant on the surface of the layer tobe lifted after the channels are made (FIG. 6a) but prior to theaffixing of the support (FIG. 6c). This can be done by pouring theliquid onto the surface (FIG. 6b), or submerging the sample duringapplication of the support material 80.

The tape's adhesion can then be released by UV irradiation through thealumina (FIG. 7b) and the tape can be taken off the alumina carrier withthe devices still attached. Further UV exposure will decrease theadhesion of the devices to the tape, allowing them to be removed byvacuum wand or to be transferred directly from the tape to any othertape, epoxy, or adhesive medium (See FIGS. 7b or 7 c). FIG. 7b showsrelease directly to another tape 110 a whereas FIG. 3c illustratesrelease by affixing with epoxy 15 to a rigid substrate 25. Discreteareas as large as 0.5 cm in width have been lifted, released andtransferred by this non-curvature method. The total wafer area which canbe lifted simultaneously in a registered fashion by way of this channelmethod appears to be limited only by the wafer size.

Along the same lines, channels 60, as shown in FIG. 5a, can be made inthe LED material 16 to expose the release layer 14 below. A porousmaterial 90 is then spun on, or otherwise formed or attached to thefront surface (FIG. 5a). This material is rigid or semi-rigid when curedby light, heat, or solvent release, and therefore will be able tosupport the lifted film (FIG. 5b) after separation from the substrate12. The material is sufficiently porous to pass the etchant fluidwithout being altered by the etchant. In this way, the etchant passesthrough the porous material and is given access to the release layer atits exposed points.

The channel method and porous material method for liftoff allow for asolid support medium to be employed without requiring perforations foretchant access. The liftoff time is very short because the etchant hasaccess to the release layer from many points on the wafer surface. Also,this method results in devices which are registered with respect to eachother and are supported by the alumina or other rigid material forbackside processing.

II. LEDs and LED Bars

Formation of LED's or LED Bars having a unique thin film structure andother features which provide a highly efficient light emitting structurewill now be described in connection with FIG. 1 in which an LEDsemiconductor-layer structure 16 is formed on release layer 14,preferably by OMCVD. Note that while FIG. 1 has previously been used todescribe lift-off procedures, the lift-off method is considered to beoptional, but preferred in this discussion. Structure 16 is formed ofmaterials having appropriate bandgap for the desired emissionwavelength, such as III-V materials. Structure 16 is preferablycomprised of an n-doped upper cladding layer 16 a, an active p-dopedlayer 16 b and a lower p-doped cladding layer 16 c (see FIG. 1e).Preferably, the cladding layers 16 a and c are formed of III-IVmaterials, such as Al_(x)Ga_(y)As, (y=x−1) and the active layer 16 b ofAl_(z)Ga_(1−z)As. A contact layer 16 d, preferably of GaAs, is alsoformed on the top (light emitting surface), of upper cladding layer 16 aand an optional bottom contact layer 16 e is provided beneath layer 16c.

Note: As a matter of convention, the light emitting side of the LED willgenerally be referred to as the “front side” herein. Also, theconductivity of the layers may be reversed, such that, the upper layers16 d and a are p-type and the lower layers 16 b and c are n-type.

By using OMCVD and by using the above-mentioned isolation systems, theLED structure 16 can be made very thin, i.e., less than about 5 micronsand, preferably, less than 3 microns, with the contact layer 16 d beingless than 0.1 micron thick, the cladding layers 16 a and c about 2microns thick and the active layer 16 b less than about 1 micron.

A section of an LED pixel formed as above is shown in FIG. 1f with thecontact structure added. For more efficient operation, the claddinglayer 16 a on the light side of the pixel is roughened; as shown in FIG.1f to increase the probability of multiple bounces of light rays withinthe LED and thereby increase the probability of achieving a good exitangle. Note: The roughened surface can be on the bottom side instead ofthe front side. Also the LED structure 16 a, b, c is a thin transparentdouble heterostructure and the substrate is removed and replaced by alight reflector layer 13 of material such as Al or a white diffusedsurface such as MgO on white ceramic, for improve quantum efficiency.Finally, unlike the conventional cubic geometry of discrete LEDs, theLED of FIG. 1(f) has a rectangular geometry with a high ration of widthto thickness. For example, the width W preferably ranges from about 50microns to about 150 microns while the thickness T is about 5 microns orless for a 10 or 20 to 1 ratio of W/T. The pixel length L is generallyabout 200 to 400 microns.

Contacts 33 and 31 are formed on the front contact layer 16 d and theback contact layer 16 e respectively. Preferably the back layer contactis extended up to the front surface to form a planar structure. Apassivating, isolating spacer 37 of Si₃N₄ or SiO₂ is formed between theLED structure and contact 31. Contact surface 39 may be formed of n⁺GaAs if contact layer 16 e is an n-type layer. Optionally layers 16 a, band c may be stepped at the edges and a conformal coating of Si₃N₄ orSiO₂ deposited over the edges.

Although the above description was made using AlGaAs and GaAs layers,the invention can be applied to other III-V lattice-matched materialsystem; for example, the active layer 16 b can be replaced withGa_(x)In_(y)P, which is lattice match with GaAs and GaAlAs, and whichhas better light emitting properties. For lattice-matching x should beroughly equal to y. The cladding layers may also be replaced withAl_(x)Ga_(z)In_(y)P. For lattice matching to GaAs, x+z=y. Finally, theactive layer GaInP can be replaced with Al_(x)Ga_(z)In_(y)P as long asit can be lattice-matched to GaAs. Such LEDs, with AlGaInP compounds,can range in emission wavelengths from about 0.55 μm to 0.70 μm.

Note that the active layer 16 b is doped with a p-type dopant,preferably carbon, during the OMCVD process. Zinc, the conventionalimpurity used for p-type doping in LEDs, is typically introduced bydiffusion after the growth process and is highly diffusive over the lifeof the LED. Consequently, the pixel or dot location may vary due to suchdiffusion. Carbon is very non-diffusive, leading to greater uniformityin dot location and longer device life. Therefore, carbon-doping in theactive layer of LEDs is much preferred.

The Al content “x” may be varied for different devices, but the claddinglayers of AlGaAs should be as heavily doped as reasonable. The Gacontent “y” should be less than “x”. Also, the Al content, z, of theactive layer should be less than x. Usually both cladding layers havethe same Al content, but that is not necessary.

The above-mentioned structures are excellent for LED discrete devices aswell as LED bars. Referring now to FIGS. 8-12, further embodiments ofthe invention will be described in connection therewith. FIGS. 8, 10 and11 summarize the important steps of three alternate processes forfabricating LED bars in accordance with the invention. Beneath each stepis the corresponding wafer structure shown in side view.

Each LED bar 200, when processed, as shown in the plan view of FIG. 12b,consists of a die 200 measuring about 0.5 mm by 20 mm cut from a wafer240, as shown in FIG. 12a, upon which several hundred such dies arefabricated in accordance with FIGS. 8, 10 or 11. Each die or bar 200 iscomprised of a row of many microscopic, laterally isolated, LEDs 210,each LED forming a pixel of light or a “dot”. Each LED 210 in a row isaddressable from its own bond pad 220 connected by conductor 119 to apixel contact area.

Note that while the invention is explained herein in connection with LEDBars it is contemplated that the individual LEDs 210 may be diced andseparated along with each respective bond pad, coupled by conductor 119,to form discrete LEDs of unique configuration, in that, the bond pad isformed beside the LED and is therefore highly accessable and does notneed to be wire bonded to the LED pixel.

The main steps in LED bar fabrication are:

a) epitaxial growth to form the required LED epitaxial layers 16 and p-njunctions therein, as described in connection with FIGS. 1-7;

b) dot definition to delineate the edge of the LED spot;

c) front side metallization for contacts;

d) optional lift-off procedure as previously described in connectionwith FIGS. 1-7; and

e) backside metallization.

Only steps b, c and e will be described in detail herein, it beingassumed that any of the lift-off methods and growth methods previouslydescribed can be used in connection with the steps described herein toform registered or unregistered LEDs or LED bars.

Referring now to FIG. 8, a mesa isolation method of dot definition isshown therein. Note: for each process step block, the correspondingstructure is illustrated in section below. Step a) comprisespre-epitaxial cleaning of wafer 12 using well known techniques, such assoaking in H₂SO₄/H₂O₂ and H₂O, followed by OMCVD deposition ofAlGaAs/GaAs epi-layers 16, in which a p-n junction is formed in theactive GaAs layer (Step b).

Next, using well known photolithography techniques, individual dotjunction areas 40 are defined over the surface of epi-layers 16 beneathareas of photoresist 105 (Step c). Next, the exposed epi-layers 16 areetched away down to just below the p/n junction or alternatively all theway down to substrate 12 (Step d). The resist 105 is removed and aprotective coating 106 of Si₃N₄ or oxy-nitride (SiON) is formed over thetop surface (Step e). Contact areas 171 are photolithographicallydefined by resist 115 over the nitride 106 (Step f). The nitride 106 isetched away beneath the resist openings (Step g). The resist is strippedaway and a “lift-off” photo-resist layer 117 is formed over the topsurface, except where the metal contacts will reside (Step h). Frontmetallization layer 119 is evaporated onto the resist contacting theexposed epi-layer surface aligned in the LED dot (Step i).

The resist 117 with metallization 119 is then removed using well-knownphotoresist stripper liquids, leaving metal contacts 119′ remaining andapplied to each dot 16 (Step j). These contacts extend over the nitride116 to the edge of the chip (See FIGS. 9 and 12) where individual bondpads are formed to address each dot 16′. Contact metallization 121 isthen applied to the back of the substrate 12.

FIG. 10 illustrates an alternate dot definition method utilizing ionbeam implantation. Steps a and b are as set forth in connection withFIG. 8. In step c, an implant mask of photoresist 105 is formed whichdefines regions 41 between LEDs which will be ion bombarded to implantprotons 111 (Step d) to laterally isolate individual dots or pixels 16′,separated by highly resistive bombarded regions 41′ (See FIG. 10 notes).Next (Step e), a lift-off photoresist layer 115 is formed on the exposedtop surface of epi-layers 16 with openings left where contactmetallization 119 will be evaporated (Step f). The metallization isremoved everywhere, except where defined, to form individual contacts119′ for each dot 16′. Contact metallization 121 is then applied to thebackside (Step h).

FIG. 11 depicts an alternate dot definition process that doe snotrequire a separate deposit of a dielectric layer with associatedphotolithography, as in FIG. 8. Steps a-b are as above. In thisalternate method, after defining the dot edges (Step c), the cap orcontact layer 16 d [discussed in connection with FIG. 1(e)] is etchedaway (Step d). The exposed epilayer surface 16 is then anodized to forman insulating oxide 108, thus creating a dielectric in the properpattern. This method, as in the method of FIG. 8, limits currentspreading to the pixel area where it is desirable for uniform currentinjection. But, by removing the cap layer from regions between dots,illumination within the confines of each dot is maintained. Currentspreading is further eliminated by growing an extremely thin uppercladding layer 16 a, which will have very high lateral resistivity.Conventional cladding layers are 20 microns or higher. OMCVD enablesfabrication of 0.5 micron, or less, layers with 0.2 micron being apreferred thickness for layer 16 a.

The resist 105 is then removed (Step f) and a photoresist layer 115formed, except where contacts are desired. Metal 119 is evaporated overand between the resist (Step h) and removed (Step i) leaving contacts119′ to each dot 16′. The structure is then ready for back metallization121, as previously described in connection with FIG. 8 (Step j).

In a variation of FIG. 11, the cap 16 d and cladding layer 16 a couldboth be anodized, eliminating the need for a cap etch step.

The above processes offer many advantages over other known systems offabricating LEDs or LED bars. Some of these are the following:

Lattice-Matched System. The epitaxy process is very nearly perfectlylattice matched, since it is made in the GaAs/AlGaAs system rather thanthe GaAs/GaAsP system. Thus, compositional grading to achieve latticematching is not required. The epi-layers are thin (less than 3 microns)as opposed to 20 to 30 microns in the GaAs/GaAsP system. Since thelayers are thinner and are made by OMCVD, the layers yield much moreuniform electroluminescence, making the LED bar more uniform. Since theepitaxial layers are lattice matched, it is also a simple matter tochange the process to grow LEDs of any wavelength in the range of about650 nm to 870 nm. The above processes can also utilize GaInP for theactive epi layers and AlGaInP for the cladding layers. Another possiblelattice matched system is GaInAsP/InP.

Better Confinement of Injected Carriers. The beneficial properties ofAlGaAs layers can be used to enhance the optical output of the LEDdevices, in a manner similar to heterojunction lasers. The AlGaAs isused to reflect carriers so that they are confined to the volume inwhich the optical radiation is to be generated. This enables thegeneration of much higher efficiency and optical output than is believedto be possible in the GaAs/GaAsP system.

Epitaxially-grown P/N Junction. The junctions are grown during the OMCVDprocess. In general, in GaAs/GaAsP technology, the junction is diffused.The epitaxial junctions are of extremely high quality and can be placedanywhere in the structure. Diffused-zinc junctions used in GaAs/GaAsPhave the following limitations: the zinc causes p-type doping, so thestructure must be p-on-n (whereas epitaxial junctions can be p-on-n orn-on-p); the zinc concentration must be highest at the surface and musthave a diffusion profile (whereas epitaxial doping can have anyprofile), the diffused junctions are limited to zinc (whereas epitaxialstructures can be zinc, or carbon, or other dopant as desired).

Implant Isolation. In the FIG. 10 embodiment, the epitaxial wafers areimplanted with protons to destroy the crystal quality of the regionsbetween the dots. This isolation is used to prevent the current fromspreading beyond the desired dot perimeter. (The GaAs/GaAsP technologyuses patterned diffusion.) An additional advantage of implant isolationis that the surface becomes nonconducting so that the metallization canbe placed directly on the semiconductor, without dielectric insulators,and no short circuit will occur.

Use of GaAs Cap. A very thin layer 16 d, about 1000 Å thick, of GaAs isprovided on the top surface for three reasons: ease of contact,environmental stability, and improvement in current spreading. The GaAsis kept thin to allow most of the generated light to escape. If the capis much thicker than 1000 Å, it will absorb a significant amount oflight. Environmental stability is a factor because AlGaAs can oxidize inair if left uncoated. The GaAs cap 16 d provides the required coating.

LED Bars fabricated as described above may be modified as shown in FIGS.23-26 to incorporate a cantelevered contact bar 240 which mates with acorresponding contact bar 242 on a processed silicon wafer 260. In thisway, a hybrid Si/LED structure can be formed with a minimum of wirebonding and avoidance of alignment problems.

Contact Bar 240 is bonded or otherwise affixed to the front side of LEDbar 270 and contact wires 219 extended to each pixel 261. Back contacts280 are formed on the back of LED Bar 270.

A eutectic alloy 290, of, for example Au_(0.8)Sn_(0.2) is formed on theside walls of the wafer 260 and/or the back of LED Bar 270. The bar 270and wafer are joined so that the contact bars 240 and 242 overlap. Thejoined structure is heated to the melting point of the eutectic (i.e.about 252° C.) and allowed to cool to room temperature thereby bondingthe contact bars and structure together. The contact bars may then belaser trimmed or etched or scribed to form bonding pads 250,252 (shownin dotted lines) for interconnecting Si circuits 292 to specific pixels.Si circuits 292 may comprise Si transistors connected to form drivercircuits for energizing the individual LED pixels 261.

III LED X-Y Arrays

Next, the fabrication of an X-Y multiplexed array, in accordance withthe invention, will be described. It begins with the epitaxial growth ofthe required hetero-epti-layers of AlGaAs and GaAs layers on a GaAs orGe substrate. In the case of the GaAs substrate 12, an optional layer 14of AlAs is formed between the active AlGaAs layers 16 and the substrate12 to facilitate substrate removal by the etch-off method. The AlAsforms an etch stop layer. [Alternatively, the X-Y array can be removedfrom the substrate by a CLEFT process (See U.S. Pat. No. 4,727,047issued Feb. 23, 1988 to Fan et al.) or chemical epitaxial lift-off]. Inthe case of Ge substrates, a layer of AlAs can be used as an etch stop,but AlAs is not really necessary, since the Ge substrate can bedissolved in H₂O₂ without harm to the AlGaAs active layers. FIG. 14ashows the epitaxial layer structure to comprise a bottom cladding layer16 c of AlGaAs, an active GaAs (or AlGaAs) layer 16 b in which a p-njunction 17 is formed by carbon doping during growth, a top claddinglayer 16 a of AlGaAs and thin GaAs contact layer 16 d, all, aspreviously described, formed by OMCVD. A pattern of contact pads 119 andbusbars (not shown) is formed by photolithographic techniques,evaporation, and/or electroplating on the front surface, as shown inFIG. 14b. Next, the p/n junctions 17 are isolated by etching part wayinto the epi-layers 16, as shown in FIG. 14b. This step is notabsolutely required at this point, however, it simplifies a later etchstep in the process.

The next stage of the process consists of bonding of the wafer to asupport 80, such as glass, ceramic, or thin stainless steel. (If thesupport is transparent to infrared radiation, downstream front-to-backalignments are facilitated, but the alignments can also be carried outby careful registration to the support edges.) The processed front sideis bonded to the support 80 using a suitable adhesive (not shown) (FIG.14c). After the support 80 is attached, the wafer or substrate 12 isetched off (or cleaved off) leaving the LED film 16 attached to thesupport 80, as shown in FIG. 14d, in which the structure has beenflipped over onto the support to expose the backside B for processing.

Once the backside is exposed, any remaining non-essential material isremoved from the back by selective etching in HF to expose a clean GaAscontact layer B. The backside (X-axis) contacts 121 and busbars 121 aare now photolithographically patterned and electroplated or evaporatedonto the contact regions 16′.

Finally, the backside is exposed to the mesa etch to totally separatethe dots. At this point, all of the epi-material between the pixels 16′is removed (FIG. 14e). Alternately, the isolation may be completed byimplant isolation, or by limiting the current spreading, as describedfor LED bars in connection with FIGS. 8, 10 and 11. By not removing allof the interpixel material, a path for lateral heat flow out of thepixel is preserved.

As shown in FIG. 15, the front and backside processed X-Y array 300 maybe mounted directly to silicon wafer 323 in a precise location 310 withX and Y silicon driver circuits 320 and 322 formed in wafer 323 andcoupled to the X and Y bonding pads 324 and 326, respectively. Bondingof array 300 to wafer 323 may also be accomplished as describedpreviously in connection with FIGS. 23-26 by having the contact pads 326replaced by cantelevered bars which extend over to pads on wafer 323which can be trimmed to form circuit bonding pads.

Suitable silicon logic circuits 330 and interface circuits 332 areformed on wafer 323 to control which pixel 16 is illuminated in the X-Ymatrix. Note that the driver circuits activate individual pixels byapplying a positive voltage to a pixel in a top column, for example,pixel 1601 via bus bar 326 a, while a negative voltage is applied to thesame pixel 1601 via Y-driver 322 to bottom bus bar 324 a, thuscompleting the current circuit through the LED 1601.

It should be noted that the substrate removal methods for fabrication ofLED arrays include CLEFT, lift-off, and substrate etch-off. CLEFT andlift-off are appropriate if the substrate is to be reclaimed as a solidwafer. The etch-off process simply comprises the chemical dissolution ofthe substrate. Note that the substrate material may still be reclaimedin the etch-off process; however, it must be precipitated from the etchsolution. The substrate can also be lapped off, as is conventionallydone in the industry.

Also note that in the first step of the backside process, undesiredepitaxial layers are removed; these layers are present to initiate theepitaxy, or may be buffer layers that are not needed in the finaldevice. To make their removal simple, an AlAs etch stop layer (notshown) may be provided in the epitaxy between these layers and theepitaxial device structure. The layers can then be removed in etchesthat stop at AlAs, such as the well known PA etches. At a pH of about 8,these etches dissolve GaAs 1000 times faster than AlGaAs. After the etchstops at the AlAs, the AlAs can be removed in HF or HCl.

In the process described above, the backside of the substrate isprovided with multiplex-compatible metallization to contact the back ofeach pixel. Note that this type of processing requires front-to-backalignment. The pixels are then separated by a mesa etch. Since the filmsare only about 5 microns thick, the mesa etch is straightforward andquick. The etching may be accomplished with either wet or dryprocessing. At this point, the exposed semiconductor may be coated witha dielectric to prevent oxidation.

Finally, the wafers are formed into individual dice. The dice 300 (SeeFIG. 15) are mounted in a pin grid array (PGA) or leadless chip carriersocket (neither shown). If the pixel count is sufficiently high (>1000),the X-Y drivers 320, 322 and logic multiplexing circuits 330 should bemounted within the chip carrier. The reason for this is that the wirecount becomes excessive for high pixel numbers. The wire count isapproximately the square root of the pixel count. Preferably, the arrayis mounted on the Si circuitry itself, and interconnected using thinfilm techniques and photolithographic processing. The circuit and arrayare then mounted in the leadless chip carrier or PGA.

As shown in FIG. 13, reflection from the back surface may be used toenhance emission. FIG. 13 is a perspective view of an LED array pixelshowing the upper and lower cladding layers 16 a and 16 c with theactive layer 16 b between them. Thin contact layers 16 d and 16 e areformed on the front and back sides, respectively, and conductors 119 aand b run orthogonal to each other on the contact layers. The backsurface contact layer 16 e of GaAs extends across the total pixelsurface and serves as a back surface reflector. The back surfacereflector reverses the light propagating toward the back of the pixel,so that it is directed toward the front surface. The back surface 16 emay also serve to scatter light into the escape cone; which is a rangeof angles that rays, propagating within the LED crystal, must fallwithin for the ray to propagate beyond the semiconductor/air interface.

Tuning of individual epi-layers may also be provided to further improveLED efficiency. For example, assume a structure, such as the LED shownin FIG. 13, in which the epi-layers have the following properties:

Refractive Wavelength Composition Layer Index λ/n(Å) AlGaAs AIR 1 6500N/A 16d 3.85 1688  0    16a 3.24 2006 80% 16b 3.60 1806 38% 16c 3.242006 80% 16e N/A N/A Metal

The active layer 16 b, could be made “resonant” by making the activelayer thickness a multiple of half the wavelength (i.e., a multiple of903 Å). For example, an active layer thickness of 4510 Å or 5418 Å wouldbe preferable to 5000 Å. Such a resonant structure could yieldsuperluminescence or stimulated emission which would enhance the opticaloutput. A benefit of stimulated emission in the resonant structure wouldbe that all of the light thus generated would be in the escape cone.

The front (top) cladding layer 16 a is set for maximum transmission(quarterwave or odd multiple). The quarterwave thickness is 503 Å,therefore the top layer should be 0.55 microns, or if better currentspreading is needed, 1.05 microns.

The back cladding layer can be tuned for maximum reflection by usingeven multiples of 503 Å, such as 10×503 or 5030 Å.

Optional front and back Bragg reflector layers 16 f and 16 g,respectively, may be incorporated into the device of FIG. 13 duringOMCVD growth, thereby converting the LED into a vertical cavity laser.The laster cavity is bounded by the Bragg reflectors 16 f and 16 g andthe emitted light will be phase coherent. The Bragg reflectors areformed by alternating many Al_(x)GaAs/Al_(z)GaAs layers. A sufficientnumber of layers will yield a high reflection coefficient. Theelectrical cavity is formed by the AlGaAs cladding layers. Thus,vertical cavity lasers can be in an X-Y array, or may be formed in alaser bar. The feature that makes this possible is the double-sidedprocessing approach, which permits a wide range of pixel structures,including LEDs, lasers and detectors.

A light detector array 450 can be formed in a similar manner. To form alight detector array, the epitaxial films are doped so as to form ap-i-n structure, rather than an LED. The active layer comprises asemiconductor chosen for absorption over the wavelength range ofinterest. For example, long wavelength detection could utilize InAsgrown on an InAs substrate. Alternatively, InGaAs grown on InP or GaAscould be utilized for mid-IR detection. Near IR is detected with GaAs orAlGaAs. The fabrication of the detector must include edge passivation tomaintain minimal dark current, but is otherwise the same as the LEDarray processing previously described.

The multiplexing electronic detector circuitry is somewhat differentthan the LED driver circuit, since it must sense the current generatedin each pixel in sequence, rather than supply current. The electronicsis nevertheless straightforward, and is similar to charge coupled device(CCD) circuitry. In fact, the device could be formed using a CCD arrayinstead of a p-i-n array.

An infrared-to-visible digital image converter can be formed from adetector 450 and light emitting diode array 300 (as shown in FIG. 19).The converter is useful for night vision devices, as well as for digitalprocessing of IR and visible video data.

Current image converters utilize a photocathode-based system thatconverts IR radiation to visible. The conversion process is a directanalog process. Owing to this design, the direct analog process is notparticularly amenable to digital image enhancement. There are alsovarious displays that could be superimposed over the night visiondisplay to provide the user with communication or computer data.However, the photocathode display is not easily adaptable to displayapplications.

A digital pixel-based system, in accordance with FIGS. 18 and 19,functions both as an IR image converter, an image enhancing device, anda display.

The converter invention consists of three main elements: the IR detectorarray 450, the multiplexing electronics 470, and the light emittingdiode (LED) array 300. A diagram of the IR image converter is shown inFIG. 19. An IR image is focused by lens 460 on a multiplexed X-Y array450 of IR detectors. The pixel data from the detectors is processed bythe electronics 470, which then drives a synchronous multiplexed LEDarray 300. Note that the processor can accept external data via dataport 472 to add to or subtract from the image. In this way, imageenhancement can be accomplished, or communications or other data can besuperimposed on the display 300.

As noted above, the detector array 450 can comprise a Si charge coupleddevice, or if longer wavelength detection is required, can be made fromp-i-n diodes formed from material in the InGaAs system. The array 450 isfabricated using substrate etch-off or lift-off processing, along withbackside processing, to form very thin structures with metallization onboth sides, as more fully described above in connection with the LEDarray 300.

The intensity of the image produced by array 300 may be controlled byvarying the duty cycle timing or modulating the drive current of the LEDpixels.

The electronics 470 consists of a multiplexing and sequencing circuitthat first detects the charge or current in each IR detector, and thencouples this input data to a current amplifier that drives thecorresponding LED pixel in the output array 300. The electronicprocessor 470 also accepts signals from an external source, such as amicroprocessor that can be displayed on the LED array. Moreover, theelectronics can supply that video data to the microprocessor for imageenhancement and can accept a return signal to be displayed on the LEDarray 300.

The LED array consists of multiplexed thin film LED pixels formed frommaterial in the AlGaInP family, and more particularly, AlGaAs for brightred displays. The array is formed using the previously describedprocessing array steps. The pixel size can be as small as 25 micronssquare and, consequently, the display can offer extremely highresolution or alternatively, fairly low cost.

As shown in FIG. 20, the detector 450 and LED array 300 can be stackedinto a hybrid assembly comprised of a top thin film IR X-Y detectorarray 450 affixed by light transparent glue to lower thin film LED array300 mounted on glass substrate 620. A glass lens 460 is affixed to thetop surface of detector 450 and heat transfer openings 460 provided asnecessary for cooling purposes. The entire structure can be quite thin(1 mil), with the electronics 470 provided around the periphery.Ultimately, the monolithic thin array can be mounted on ordinary glassesfor image enhancement of visible light, as well as for display of datasuperimposed on video images.

The applications of the device of FIGS. 18-20 include military nightvision systems, range finders, advanced military avionics, personalcommunications systems, and medical systems in which real-time imageenhancement is useful.

As shown schematically in FIGS. 16 and 17, X-Y arrays can also be usedto form a multicolor display. To make such a display, individual X-Yarrays labelled LED1, LED2 and LED3, are formed from two or moredifferent epitaxial structures. The primary difference in the structureis in the active layer material 161, 162 and 163, which must havedifferent band gaps to create different colors. For example, red 163 canbe created with AlGaAs, and green 162 can be created with InAlGaP. Thetop device Led1 may be a blue LED formed of II-VI material, such asZnSe, ZnSSe or a group IV alloy such as SiC.

The arrays must be stacked with the larger bandgap LED1 closer to theobserver. The material with the larger bandgap will be transparent tothe radiation from the smaller bandgap. Thus, in this way, the observerwill be able to see both colors.

The creation of the stack of three LEDs 1020 is as follows: First, thethree separate LED arrays LED1, LED2 and LED3 are formed, as previouslydescribed. Next, they are stacked together with glass 600 between them.

Transparent glue or epoxy 400 is used to bond the stacks on top of eachother. The upper and lower bonding pads P1 and P2 on each LED arelaterally staggered with respect to other LEDs, so that individual LEDpixels may be addressed (See plan view FIG. 17).

An LED array, made in accordance with the above-described methods, canalso be provided behind a lens or pushbutton, so that a message may bedisplayed. The message can be used to label the function of thepushbutton. The label and function may be assigned by a microprocessor.

Keyboards, pushbuttons and the like now dominate the interface betweenadvanced microprocessor-based electronic instrumentation and the user.Commercial electronics and computers, such as work stations, avionics,telecommunications centers, and other advanced electronic systems, arelimited by the functionality of the keyboard and button based interface.Even consumer and automotive electronics are beginning to reach thelimits of the user/microprocessor interface.

For example, typical PC keyboards now employ as many as 20 re-assignablefunction keys, the purpose of which changes with each software package.It is not uncommon to see keyboards with numerous assignment stickers,templates, or other means of tracking the particular assignments of thekeys. Even the alphabetical keys have numerous assignments. Obviously,the functionality of the various programs is becoming limited by theuser's ability to handle numerous registers of multifunction keys. Inother electronic instruments, a similar problem exists. Modern medicalinstrumentation, for example, utilizes buttons that are positioned nextto a cathode ray tube (CRT). The CRT is used to label the function ofthe buttons. The software determines the function of the button and thelabel to be displayed on the CRT, hence, the term “soft-key” has beenused to describe this type of re-assignable button.

In many applications, buttons cannot be placed next to a CRT; thepersonal computer is a good example of such an application(nevertheless, some software packages label the buttons this way, butmost users dislike the arrangement). In such a case, what is needed is abutton that has an internal display capable of providing a label with amessage provided by the resident active software.

In accordance with the invention, an X-Y LED array 966 is mounted withina pushbutton, as shown in FIG. 21. The X-Y array matrix is formed aspreviously described above in connection with FIGS. 14-17. The matrix iscapable of providing two five letter words 980, or more, such as“DELAY”, to identify the reassignable function of the pushbutton 960.The X-Y LED matrix 966 may be quite small, in order to be manufacturedat a low cost. In such a case, the pushbutton also includes a small lens964 that magnifies the image. Ideally, the electronics 968 that drivethe X-Y array are located within the leadless chip carrier that housesthe X-Y array 966. However, the electronics can also be located behindthe array within the switch housing 970.

The front lens 962 or rear-projection screen, which also serves as thebutton, is mechanically attached by plunger 972 to a mechanical switch(not shown), so that when the surface of lens 962 is depressed by theuser, a signal may be sent to a microprocessor (not shown).

Another embodiment which provides a wider viewing angle comprises theprojection of the X-Y image onto a rear-projection screen; in such acase, the lens is positioned between the screen and the matrix. Thescreen is made of a plastic that transmits red light, sot hat theinternal parts of the button are not visible, but which neverthelesstransmits the image from the LEDs.

The smart switch button or message center should have numerousapplications in advanced instrumentation and electronics. One mainapplication will be in workstations in which a large number ofreassignable function keys are needed. The second main application is oninstrumentation in which front panel space is limited, such as inmedical electronics and in avionics.

Equivalents

This completes the description of the preferred embodiments of theinvention. Those skilled in the art may recognize other equivalentembodiments to those described herein; which equivalents are intended tobe encompassed by the claims attached hereto. For example, while anOMCVD process is preferred for the reasons given above, molecular beamepitaxy (MBE) and chemical vapor deposition (CVD) and chemical beamepitaxy (CBE) based processes are also envisioned. Likewise, other typesof material removal processes, in addition to chemical etching, such asreactive ion etching, are contemplated. Also, while an GaAs active layerhas been described an AlGaAs layer can also be used wherein the aluminumpercentage may vary from 0-38%.

What is claimed is:
 1. A light emitting video display device comprising:a plurality of light emitting pixel elements formed from a lightemitting material, the pixel elements positioned to form an array ofcolumns and rows of an integrated circuit video display device, eachpixel element having an area of less than 30 μm²; a column drivercircuit formed from a semiconductor material and monolithicallyconnected to the columns of pixel elements; a row driver circuit formedfrom a semiconductor material and monolithically connected to the rowsof pixel elements; a common transparent substrate to which the array ofpixel elements, the column driver circuit and the row driver circuit arebonded with a bonding layer; and a display control circuit connected tothe column driver circuit and the row driver circuit, the displaycontrol circuit selectively actuating the pixel elements to displayvideo images.
 2. The light emitting video display device of claim 1further comprising a first plurality of pixel elements emitting at afirst wavelength and a second plurality of pixel elements emitting at asecond wavelength to provide a multicolor display.
 3. The light emittingvideo display device of claim 1 further comprising a support that mountsthe display device on a user's head.
 4. The light emitting video displaydevice of claim 1 wherein the light emitting material comprises galliumarsenide.
 5. The light emitting video display device of claim 1 whereinthe row driver circuit an the column driver circuit are formed from asilicon structure.
 6. The light emitting video display device of claim 1wherein the control circuit comprises a microprocessor.
 7. The lightemitting video display device of claim 1 wherein there are more thanabout 1000 pixel elements.
 8. The light emitting video display device ofclaim 1 wherein the column driver circuit and the row driver circuit areformed from a common layer of semiconductor material.
 9. The lightemitting video display device of claim 1 wherein the common substrate isa silicon substrate.
 10. A light emitting display device comprising: aplurality of light emitting pixel elements formed from a light emittingmaterial, the pixel elements positioned to form an array of columns androws of a display panel, each pixel element having an area of less than30 μm²; a column driver circuit formed from a layer of semiconductormaterial and connected to the columns of pixel elements; a row drivercircuit formed from the layer of semiconductor material and connected tothe rows of pixel elements; a common optically transparent substrate towhich the array of pixel element, the column driver circuit and the rowdriver circuit are bonded with a bonding layer; and a display controlcircuit connected to the column driver circuit and the row drivercircuit, the display control circuit selectively actuating the pixelelements to display an image.
 11. A light emitting display devicecomprising: a plurality of light emitting diode (LED) pixel elementsformed from a III-V material, the pixel elements positioned to form anarray of columns and rows of a display panel, each pixel element havingan area of less than 30 μm²; a column driver circuit formed from a layerof silicon semiconductor material and connected to the columns of pixelelements; a row driver circuit formed from the layer of siliconsemiconductor material and connected to the rows of pixel elements; anoptically transparent substrate to which the array of pixel elements,the column driver circuit and the row driver circuit are mounted with anadhesive material; and a display control circuit connected to the columndriver circuit an the row driver circuit, the display control circuitselectively actuating the pixel elements to display an image.
 12. Thedevice of claim 11 wherein the substrate comprises glass.
 13. The deviceof claim 11 wherein the device comprises a personal communicationsdevice.
 14. The device of claim 11 wherein the device comprises atelephone.
 15. The device of claim 11 wherein the column driver and therow driver are connected to a microprocessor circuit.
 16. The device ofclaim 11 further comprising a display input circuit formed with thelayer of semiconductor material.
 17. The device of claim 11 furthercomprising a lens to magnify an image generated on the display.